A capacitor-discharge-type ignition device for an internal combustion engine comprises an ignition coil, an ignition capacitor provided in a primary side of the ignition coil and charged in one polarity by an output of an ignition power supply, a discharge switch which becomes a conducting state when receiving an ignition signal and causes electrical charges stored in the ignition capacitor to be discharged through the primary coil of the ignition coil, and an ignition control unit for supplying the ignition signal to the discharge switch at an ignition timing of the internal combustion engine. As the ignition power supply, exciter coils provided in magneto AC generators mounted to engines are often used.
For current vehicles driven by internal combustion engines and apparatuses driven by internal combustion engines, it is required to control an ignition position of an engine (a crank angle position at which ignition is performed) in a complicated manner depending on various control conditions including a rotational speed of the engine in order to reduce noise caused by the engine, purify exhaust gas, or provide efficient operation. Because of this, ignition devices having an ignition control unit with a microprocessor are used, even in internal combustion engines that require cost reduction.
When controlling an ignition timing using a microprocessor, information on a particular crank angle position of the engine is obtained in any manner; a rotational speed of the engine is arithmetically operated based on the crank angle position information; and an ignition position of the engine is arithmetically operated with respect to various control conditions including the arithmetically operated rotational speed.
The crank angle position information mentioned above is information that, for example, indicates that the crank angle position of the engine is at a reference crank angle position having a certain relation with respect to a top dead center position (a crank angle position when the piston reaches the top dead center). In this case, the ignition position of the engine is arithmetically operated as an angle from the reference crank angle position to the ignition position or an angle from the top dead center to the ignition position. The angle indicating the ignition position arithmetically operated is converted to ignition timing detection time data by using the rotational speed of the engine at that time. The ignition timing detection time data represents a time period required for the engine to take from the reference crank angle position to the ignition position at the rotational speed at that time (time period to be measured by a timer in the microprocessor).
The ignition control unit recognizes that the crank angle position of the engine coincides with the reference crank angle position upon occurrence of a signal indicating the reference crank angle position, then sets ignition timing measurement data into a timer for measurement of ignition timing (or an “ignition timer”), and generates an ignition signal upon completion of the measurement of the time data that has been set by the ignition timer.
As a signal source for obtaining the crank angle information of the engine, a pulser (a pulse signal generator) is used that generates a pulse signal at the reference crank angle position of the engine, while it may be required to omit the pulser when cost reduction is important.
An ignition device having no pulser, or so-called pulserless-type ignition device is disclosed, for example, in Japanese Patent Laid-Open Publication No. 2003-307171. In this pulserless-type ignition device, crank angle information is obtained from an output voltage of an exciter coil that is provided to charge an ignition capacitor. In the case of obtaining the crank angle information from the output voltage of the exciter coil, a magneto generator is comprised so that the exciter coil generates, only once for one cylinder during one rotation of a crankshaft in forward rotation of the engine, an AC voltage of a waveform, as shown in FIG. 15, that has a positive half-wave voltage Vp1 having a peak value that is adequate to charge the ignition capacitor and first and second negative half-wave voltages Vn1 and Vn2 occurring before and after the positive half-wave voltage Vp1, respectively.
In the ignition device disclosed in Japanese Patent Laid-Open Publication No. 2003-307171, the magneto generator is comprised so that the second negative half-wave voltage Vn2 is generated just before the top dead center position of the engine (a crank angle position when the piston reaches the top dead center) TDC. In this ignition device, there are employed as the ignition position in starting period, a crank angle position θi0 at the time when the magnitude of the second negative half-wave voltage Vn2 after passing the peak point is decreased to a set level Vs1, and as the ignition position in idling operation, a crank angle position θi1 just after the peak position of the second negative half-wave voltage Vn2. Also, in this ignition device, the positive half-wave voltage Vp1 is compared with a set voltage Vs2, and a crank angle position at the time when the positive half-wave voltage Vp1 is equal to the set voltage Vs2 is detected as a reference crank angle position θs. The reference crank angle position θs is a position where time data for obtaining the rotational speed of the engine is taken and the measurement of the arithmetically operated ignition position is started, and is set at more advanced position than the ignition position where the advance angle width is at maximum.
A microprocessor obtains a time measured by the timer for each detection of the reference crank angle position θs, then obtains as rotational speed detection time data, the time period from the previous detection of the reference crank angle position to the current detection of the reference crank angle position (i.e. time period required for one rotation of the crankshaft), and arithmetically operates the rotational speed of the engine from the time data. The microprocessor also arithmetically operates an ignition position of the engine with respect to the arithmetically operated rotational speed, and obtains as ignition position measurement time data, a time period that is required for the engine to rotate to the arithmetically operated ignition position at the current rotational speed, then sets the time data into the timer and causes the timer to start the measurement of the time data. The set voltage Vs2, which is compared with the positive half-wave voltage Vp1 to obtain the reference crank angle position, is set to be equal to a value near the minimum value of the charging voltage of the ignition capacitor required for desirable ignition operation.
In starting period of the engine, ignition operation is performed by generating an ignition signal when the crank angle position θi0 is detected, while in idling operation of the engine, ignition operation is performed by generating an ignition signal when the crank angle position θi1 is detected. In a steady-state operation period where the rotational speed of the engine is higher than the idling rotational speed, ignition operation is performed by arithmetically operating an ignition position of the engine with respect to the rotational speed detected at the reference crank angle position θs, converting the arithmetically operated ignition position to ignition position measurement time data, and causing the ignition timer to measure the time data and generate an ignition signal at the completion of the measurement.
An ignition device disclosed in Japanese Patent Laid-Open No. 2003-13829 is known as an ignition device in which an ignition capacitor is charged by using AC voltage of waveform as shown in FIG. 15, and rotation information is obtained including information on the reference crank angle position of an engine. In the ignition device disclosed in Japanese Patent Laid-Open Publication No. 2003-13829, the ignition position in starting period is set at a rising point or peak point of the second negative half-wave voltage Vn2 of AC voltage shown in FIG. 15, and the peak point of the first negative half-wave voltage Vn1 is detected as the reference crank angle position.
Note that the starting period of the internal combustion engine used herein means a transient period required from the initiation of starting operation, through completion of the starting operation of the engine, to reaching of a rotation-sustainable state of the engine.
When the ignition capacitor is charged with the positive half-wave voltage Vp1 in AC voltage of waveform as shown in FIG. 15, a delay of the positive half-wave voltage Vp1 occurs in high speed operation of the engine due to armature reaction. Accordingly, the angle between the reference crank angle position θs and the ignition position in starting period θi0 must be substantially large so that both the condition for obtaining a desired maximum advance angle width and the condition for setting the ignition position in starting period at a position close to the top dead center can be met. Therefore, the conventional ignition device has a problem that a large-size magneto generator is necessary, since it is required to provide large spacing between magnetic poles of a rotor and a stator in the magneto generator in order to increase the widths of the positive half-wave voltage and negative half-wave voltages generated by the exciter coil.
Moreover, in the case where, as described above, the positive half-wave voltage Vp1 of the exciter coil is compared with the set voltage Vs2 to obtain the reference crank angle position θs, it required to provide a hardware circuit having a comparator for comparing analog voltages to obtain the reference crank angle position and also required to provide a peak detection circuit for detecting a peak position of the negative half-wave voltage. It is supposed that the peak position of the negative half-wave voltage may be detected by digital process, but this requires an A/D converter, and therefore, there is a problem in either case that a complicated hardware circuit is needed leading to a higher cost.
It is also supposed that when measuring the ignition position after the engine has started, measurement of ignition position measurement time data is performed over a section corresponding to about one rotation of the crankshaft while regarding a zero-cross point or peak point of the second negative half-wave voltage Vn2 as the reference crank angle position, and after the detection of the reference crank angle position, an ignition signal is generated after about one more rotation of the crankshaft, in order to allow a large advance angle width. However, in a low speed operation of the engine, variation in rotational speed during one rotation of the crankshaft is large, and therefore, in the case where the measurement of ignition position is performed over the section corresponding to about one rotation of the crankshaft, measurement error of the ignition position in a low speed operation is large and the ignition position cannot be controlled with high accuracy. It is preferred that the angle from the reference crank angle position to the ignition position is as small as possible so that the ignition position can be controlled with high accuracy.
Moreover, in order to an ignition signal to be generated after about one rotation after the detection of the reference crank angle position, as described above, it is required to employ as the rotational speed used in arithmetic operation for obtaining the ignition position detection time data, a rotational speed of the engine detected in the section from the point of two rotation before to the point of one rotation before, so that there is large difference between the rotational speed used in the arithmetic operation for obtaining the ignition position detection time data and the actual rotational speed at the ignition timing, resulting in a poor responsibility of ignition position control against variation in rotational speed of the engine, which must make the rotation in starting period of the engine unstable.